The Avatar can. That's the soul that's born into the world in a cycle! Into the air nomads, then the water tribes, then the earth kingdom, then the fire nation! He or she is responsible for keeping peace and balance in the world.

We've got three of those here, though. See, they're all reincarnated versions of one another! ...Well, I guess technically they're pre-incarnated, too, 'cause Roku was the fire nation avatar before grandpa was the air nomad avatar before Korra was the water tribe avatar.